try to guess the codec for imag2pipe too

Originally committed as revision 3674 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/utils.c

@@ -138,7 +138,7 @@ enum CodecID av_guess_codec(AVOutputFormat *fmt, const char *short_name,
     if(type == CODEC_TYPE_VIDEO){
         enum CodecID codec_id= CODEC_ID_NONE;
 
-        if(!strcmp(fmt->name, "image2")){
+        if(!strcmp(fmt->name, "image2") || !strcmp(fmt->name, "image2pipe")){
             codec_id= av_guess_image2_codec(filename);
         }
         if(codec_id == CODEC_ID_NONE)